Investment casting may be used to produce hollow parts having internal cooling passages. During the investment casting process, wax is injected into a wax cavity to form a wax pattern between a core and a wax die. The wax die is removed, and the core and wax pattern are dipped into the ceramic slurry to form a ceramic shell around the wax pattern. The wax pattern is thermally removed, leaving a mold cavity. Molten metal is cast between the ceramic core and the ceramic shell, which are then removed to reveal the finished part.
Any movement between the ceramic core and the wax die may result in a distorted wax pattern. Since the ceramic shell forms around the wax pattern, and the ceramic shell forms the mold cavity for the final part, this relative movement may result in an unacceptable part. Likewise, any movement between the ceramic core and the ceramic shell when casting the airfoil itself may result in an unacceptable part. Specifically, cooling channels formed into a wall of the finished part require that the wall, which is formed by the mold cavity, meet tight manufacturing tolerances. As gas turbine engine technology progresses, so does the need for more complex cooling schemes. These complex cooling schemes may produce passages that range in size from relatively small to relatively large, and hence manufacturing tolerances are becoming more prominent in the design of components.
The nature of the investment casting process, where two discrete parts must be held in a single positional relationship during handling and multiple casting operations, makes holding the tolerances difficult. In addition, the ceramic core itself is relatively long and thin when compared to the wax die and ceramic shell. As a result, when heated, the ceramic core may distort from its originally intended shape. Likewise, the ceramic core may not expand in all dimensions in exactly the same manner as the wax die and/or the ceramic shell. This relative movement may also change the mold cavity and render the final part unacceptable.
In order to overcome this relative shifting, U.S. Pat. No. 5,296,308 to Caccavale et al. describes a ceramic core having bumpers on the ceramic core that touch, or almost touch, the wax die during the wax pattern pour. This controls a gap between the ceramic core and the wax die, and likewise controls a gap between the ceramic core and the ceramic shell. Controlling the gap minimizes shifting between the ceramic core and the ceramic shell, and this improves control of the wall thickness of the airfoil. The bumpers are positioned at key stress regions to counteract distortions. The final part may have a hole where the bumpers were located, between an internal cooling passage and a surface of the airfoil, which allows cooling fluid to leak from the internal cooling passage.